Recently, an increasing number of electronic equipment such as AV equipment and personal computers are becoming cordless and portable. With this development, many non-aqueous electrolyte batteries with high energy density containing a non-aqueous electrolyte, are being adopted. Of the non-aqueous electrolyte batteries, lithium secondary batteries are the ones that are most widely used in practical applications.
For the negative electrode of the lithium secondary battery, negative electrode materials capable of absorbing and desorbing lithium as well as having a low electric potential closer to that of lithium, such as graphite and amorphous carbon, are being employed. On the other hand, for the positive electrode, for example, lithium-containing transition metal compounds capable of absorbing and desorbing lithium as well as having a high electric potential, such as LiCoO2 and LiMn2O4, are being employed as the positive electrode material.
The electrode plate of the non-aqueous electrolyte battery is produced, for example, in the following manner.
Firstly, a slurry-like electrode mixture which contains a positive electrode material or a negative electrode material, a binder and a dispersion medium, is prepared. The electrode mixture is applied onto a current collector or a core material such as a metal sheet, metal mesh, metal lath sheet and punched metal, which is then rolled, dried and cut into a desired shape to give an electrode plate.
The non-aqueous electrolyte is prepared by dissolving a lithium salt such as LiPF6 or LiBF4 in a non-aqueous solvent. As the non-aqueous solvent, for example, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, diethyl carbonate and the like are employed. These days, a mixed solvent of an open-chain compound and a cyclic compound is in great use.
A conventional non-aqueous electrolyte battery comprises, for example, an electrode plate group, which is produced by spirally winding a positive electrode and a negative electrode with a separator disposed therebetween, and a cylindrical-type or square-type container for housing the electrode plate group together with a non-aqueous electrolyte. The opening of the container is sealed by a sealing plate which also serves as an external terminal. A battery having such structure is difficult to be designed thinner. However, with the recent trend for more compact electronic equipment, there is a strong demand for a small, light-weight battery with a sufficient energy density which can be accommodated in a limited space. Even batteries with a thickness of less than several millimeters are demanded in many cases.
For this reason, polymer electrolytes are recently being applied to batteries. As a polymer electrolyte, a gel electrolyte is employed, which comprises a liquid non-aqueous electrolyte and a polymer retaining the same. The polymer electrolyte has both functions of transferring ions and separating electrode plates. Thin polymer batteries with high energy density have also been developed, which are fabricated by stacking a positive electrode and a negative electrode with a separator layer containing the polymer electrolyte disposed therebetween and covering the whole by an outer jacket.
The separator layer containing the polymer electrolyte is formed, for example, by causing a microporous membrane or non-woven fabric each containing a gel-forming agent to absorb a liquid non-aqueous electrolyte and sandwiching it between electrode plates. As the gel-forming agent, a polymer capable of absorbing a liquid non-aqueous electrolyte and forming a gel electrolyte, is employed.
A separator layer composed only of a polymer electrolyte can also be formed. Examples of the method of forming such separator include the one which involves mixing a gel-forming agent and a solvent to prepare a paste, stacking electrode plates with the paste interposed therebetween and drying the whole, followed by causing the gel-forming agent to absorb the liquid non-aqueous electrolyte. Additionally, another method is known, which involves mixing a gel-forming agent and a liquid non-aqueous electrolyte to prepare a paste and stacking electrode plates with the paste interposed therebetween.
Japanese Unexamined Patent Publication No. 2000-67850 discloses a technique of integrating electrode plates with a separator layer comprising a polymer electrolyte interposed therebetween.
In each of Japanese Unexamined Patent Publication Nos. 2000-12084, 2000-156209 and 2000-223108, there is disclosed an electrode plate group, formed by disposing a pair of electrode plates which comprises a pair of current collectors and an active material layer formed on one surface of each of the current collector so as to face each other with the active material layers disposed facing inwardly, and sandwiching an electrode plate of a different polarity by the pair of the electrode plates with a separator layer comprising a polymer electrolyte interposed therebetween. There is also disclosed a battery in which an electrode plate group is accommodated in an outer jacket made of a laminate sheet comprising a resin layer and metal foil.
Japanese Unexamined Patent Publication No. Hei 11-265699 discloses a battery in which an electrode plate group having a separator layer comprising a polymer electrolyte is accommodated in a bag-like, film-type outer jacket provided with a safety venting mechanism.
Japanese Examined Patent Publication No. Hei 9-506208 discloses a battery in which a flat, spirally wound electrode plate group having a separator layer comprising a polymer electrolyte is accommodated in an envelope-like outer jacket having an external terminal.
In each of the conventional thin batteries described above, the electrode plate group is accommodated in an outer jacket which is separately prepared. There is a limit on the simplification of the outer jacket structure, the reduction in thickness and improvement in energy density of the battery, and the simplification of the battery manufacturing process, as long as these are attempted based upon the idea of using a separately prepared outer jacket.